This invention relates to electronic systems for determining the values of a number of analog sensors and more particularly to systems for rapidly and economically providing digital readings from small, precision transducer systems.
Transducers, such as capacitive, resistive, or inductive pressure transducers, generate signals in response to minute displacements of a movable element. In a pressure transducer as disclosed in U.S. Pat. No. 4,562,742, for example, the total excursion of a flexible sensing diaphragm may be no more than 0.001". This minute deflection range is used in order to achieve better linearity and lower hysteresis and must be accurately converted to analog or preferably digital readings. In this type of device, sensing and reference electrodes on the sensing diaphragm provide capacitive pairs which may be utilized to control the frequency of associated oscillators. These oscillators in turn may be utilized to generate digital counts from which precise pressure readings may be computed for automatic control or data processing purposes. The combination of transducer and signal generator are usually referred to as a "transmitter".
Some problems, however, arise in using variable frequency oscillators in systems and transmitters of this type, stemming from practical considerations of design, including physical size, resolution, power consumption, reliability and cost. Integrated circuit techniques are generally utilized in the fabrication of the oscillators and associated electronic circuits, in order to reduce size and power requirements. In the great majority of process control and instrumentation system applications, transducers are confined within protective sealed primary enclosures receiving fluids whose pressures are to be measured. The transducer electronics, including digitizing means, are incorporated in very limited space within attached secondary enclosures that are also sealed, explosion proof and vibration resistant. The transducer and electronics form a "transmitter" suitable for use in any typical industrial system.
However, closely adjacent variable frequency oscillators can interfere with each other in a number of ways, such as by cross-talk or by locking in phase at a fundamental frequency, or at a harmonic or sub-harmonic of the fundamental. The use of signal and device isolating techniques can reduce cross-talk and frequency lock-in tendencies, but at a substantial increase in cost, and even so may not completely eliminate the problem.
A common approach to the problem of oscillator interaction has been based on selecting frequencies of operation for the oscillators such that the harmonics as well as fundamentals are well spaced from each other. However, this approach increases the complexity and expense involved, and is impractical when it is desired to limit total bandwidth or to use a substantial number of oscillators. In addition, such oscillators can vary somewhat in response to temperature and other conditions, so that minor but perhaps cumulative errors can be introduced when a number of oscillators are used.
High resolution, insensitivity to temperature and static pressure changes, and low power drain are other features which must be attained at reasonable cost for the majority of practical applications. This requires high bit resolution in the coupling system, stability in the oscillator system, and circuit accuracy throughout the calibrated span of the device. Analog-to-digital converters of high resolution could be used for sensing the analog signal levels, but only at inordinate expense. Variable frequency oscillators having very high central frequencies can also be used, but require too much power as well as being too costly, while not avoiding the problem of possible frequency lock-in.
An important capability for a transmitter based on a sensitive transducer would be responsiveness, with high resolution, in any of a number of pressure ranges. Thus a differential transducer that could accurately digitize differential pressure readings at ranges of 10" H.sub.2 0 maximum, 100" H.sub.2 O maximum and 750" H.sub.2 O maximum (plus or minus) would have great value in terms of increased versatility and reduced inventory. These are typical figures used in industrial process situations. The prior art has heretofore usually required separate transducers for these applications, although more recently "smart" transmitters have been used which have enabled, through signal processing, some extension of range. However a ratio of about 1.6 is the maximum that has been achieved by "dumb" transmitters
There is consequently a significant need for an economic and reliable method and apparatus for converting small signal variations into reliable digital outputs over a wide dynamic range. There is a particular need for providing this function within the volumetric, power and cost constraints imposed on modern transducer systems.